The present invention relates to a hydraulic control valve system for controlling the operation of a hydraulic cylinder, and more particularly to such a system which has a float function which allows the cylinder to freely move in response to external forces which may act upon it.
It is well known to control an actuator such as a hydraulic cylinder or a hydraulic motor with a pilot operated control valve. The control valve will have a neutral position wherein cylinder motion is prevented and it will have extend and retract positions. It is often desirable to have a float function wherein flow from the actuator ports is allowed to flow, in either direction, from port to port or from port to reservoir. The cylinder or actuator attached to the ports can then move freely due to the external forces acting upon it. Make up oil can be pulled from return to prevent cavitation in the case of a differential area or single acting actuator. Typically, such a float function is achieved by having a float position on the main control valve. In applications where a single pilot valve is used for activation of both the retract and float positions, the available modulation range must be divided between these control modes. Critical metering resolution is then compromised in the retract mode. A float function has also been achieved through the use of an additional third solenoid operated valve which connects both actuator ports to sump when the third solenoid is energized, such as by separate float switch. This solution requires an additional solenoid operated valve. It would be desirable to achieve a float function in a way which does not require a float position on the main control valve and which does not require an additional solenoid.
It is also well known to use cross check valves between such a main control valve and the actuator ports. However, such cross check valves can become unstable in over-running load conditions or due to a drop in supply pressure. Designs which try to overcome this problem by maintaining a restriction in the return flow path during float may compromise performance due to increased pressure drops and undesirable metering characteristics.
Another common method of opening the load check is to vent the pressure cavity behind the check. This creates a force imbalance across the poppet in the direction to open it. Designs of this type, which rely on the spool-to-bore clearance to act as the seal, tend to have higher leakage rates, especially in electrohydraulic valve designs where the minimum clearance may be dictated by hysteresis requirements. It would be desirable to provide a load check valve arrangement which minimizes fluid leakage when the main control valve is in its neutral position and is not affected by a drop in supply or load pressure.